The present invention relates to the field of drilling oil and gas wells. In particular, the invention relates to a method for updating an earth model during construction of the well.
Earth models contain data which characterise the properties of, and surfaces bounding, the geological features which form the earth""s sub-surface, such as rock formations and faults. They are used to assist operations occurring in the earth""s sub-surface, such as the drilling of an oil or gas well, or the development of a mine.
The domain of applicability of an earth model varies greatly and should be considered on a case by case basis. Some earth models are applicable only in the near vicinity of a particular oil or gas well, or mine. Others may be valid for an entire oil or gas field, or perhaps even over a region such as the North Sea or Gulf of Mexico.
The data in an earth model consists of measurements gathered during activities such as the seismic, logging or drilling operations of the oil and gas industry, and of interpretations made from these measurements. The data may be gathered above, on, or below the earth""s surface.
As the duration or number of sub-surface operations increases, more data is gathered. This data can be used to amend the relevant earth model, with the aim that it should characterise the geology and properties ever more accurately. Clearly, it will be conducive to the efficiency of these operations if amendments which enhance the accuracy of the earth model are made as quickly as possible, or appropriate.
The oil and gas industry""s traditional approach to amending earth models has not had a real-time element, apart from using formation integrity, leak-off or extended leak-off tests, or equivalent circulating density (ECD) data in the case of drilling fluid (commonly referred to as xe2x80x9cmudxe2x80x9d) loss while drilling, to constrain the minimum principal in-situ stress. See e.g., Addis et al., xe2x80x98A comparison of leak-off test and extended leak-off test data for stress estimationxe2x80x99, paper SPE/ISRM 47235 presented at the 1998 Eurock Rock Mechanics in Petroleum Engineering Conference, Trondheim, Jul. 8-10 (hereinafter xe2x80x9cAddis et al., 1998xe2x80x9d). Measurement-while drilling (MWD) data, logging-while drilling (LWD) data, wireline logs, results from tests performed on core, drilling experience and perhaps other information are used to amend the relevant earth model using techniques identical to those used to generate the original version of the earth model. An example of these conventional techniques are described in a publication by Schlumberger Educational Services entitled xe2x80x98Log interpretation principles/applicationsxe2x80x99, Houston, Tex. (1987), incorporated herein by reference and hereinafter referred to as xe2x80x9c(Schlumberger, 1987)xe2x80x9d.
This traditional approach has been enhanced in order to improve the handling of very severe wellbore instability problems in the Cusiana field in Colombia. See, Last, N., Plumb, R. A., Harkness, R., Charlez, P., Alsen, J. and McLean, M.: xe2x80x98An integrated approach to evaluating and managing wellbore instability in the Cusiana field, Colombia, South Americaxe2x80x99, paper SPE 30464 presented at the 1995 Annual SPE Technical Conference, Dallas Oct. 22-25, incorporated herein by reference and hereinafter xe2x80x9cLast et al. (1995).xe2x80x9d Last describes an integrated approach to evaluating and managing wellbore instability. An earth model was constructed using data from existing wells, together with results obtained using a computational analysis tool which modelled the geological structure and the in-situ stress state. This allowed the upper and lower bounds to the drilling fluid density to be estimated, provided that the well trajectory was specified, using standard techniques.
As a well was drilled, a data acquisition programme which included cavings monitoring and measurements-while-drilling allowed the failure mechanisms of the rock formations to be characterised and as well as the identification and implementation of appropriate drilling practices. This led to faster, more efficient, well construction. The earth model was updated after a well or hole section had been completed using all available data, particularly (a) downhole images of borehole geometry, fractures, faults and bedding (b) stress measurements such as extended leak-off tests (c) results from tests made on core, and (d) four arm calliper logs, where available. Measurement-while-drilling techniques were used to characterise time-dependent hole geometry in particular hole, intervals.
The approach of Last et al. (1995) enhanced the analysis of formation instability that can be made while drilling, with the minimum horizontal stress also being updated in real-time. The understanding of the nature of the formation instability led to improved drilling practices allowing the wells they studied to be constructed much more efficiently.
However, the approach of Last et al. (1995) has many limitations, including the following. First, no constitutive parameter or component of the in-situ stress state, other than the minimum principal stress, was updated in real-time. Such parameters were updated after drilling had been completed, using measurements made by wireline tools, in addition to the data gathered in real-time. Second, LWD measurements, such as resistivity and sonic, were not extensively used. This inhibited the diagnosis of the instability mechanisms. Due to these limitations, it was not possible to quantify amendments to drilling practices, apart from updating the value of ECD above which drilling fluid is lost to the formation.
Thus, it is an object of the present invention to provide a method to increase the efficiency with which an oil or gas well can be constructed.
It is a further object of the present invention to provide a method in which amendments that enhance the accuracy of the earth model are made quickly and efficiently.
It is a further object of the present invention where one or more inconsistencies are identified to provide a method of selecting which component or components of an earth model to update.
According to the present invention the efficiency with which an oil or gas well is constructed can be enhanced by updating the relevant earth model using real-time measurements of the effective density of the drilling fluid and at least one other parameter. Updating the earth model need not occur on a continuous basis, but on a timescale appropriate to this construction process. As used herein the term xe2x80x9creal-timexe2x80x9d is defined to mean as the well or borehole is being constructed. That is, the data acquisition can occur continuously (e.g. measurement while drilling data), or at discrete times (e.g. palaeontological analysis of rock carried by the drilling fluid from the wellbore) and still be considered xe2x80x9creal-timexe2x80x9d.
A method for updating an earth model is provided that includes obtaining an earth model used for predicting potential problems in drilling of a borehole having a predetermined trajectory. Evaluations of the state of the borehole and local geological features are obtained which are based on the earth model. Data is received that has been gathered during the construction of the borehole. The evaluations are compared with a diagnosis of the state of the borehole and local geological features to identify at least one inconsistency. A component of the earth model is identified that is both related to the identified inconsistency and has a high degree of uncertainty. The selected component of the earth model is then updated prior to completing construction of the borehole using the received data.
Preferably, the evaluations of the state of the borehole and local geological features are predictions of one or more conditions under which the borehole will fail, and they are obtained by combining the earth model with the predetermined trajectory of the borehole.
Preferably, the process is repeated until a sufficient match exists between the predicted failure conditions and the diagnoses of the borehole.
Preferably, the earth model component that will be updated is selected by first identifying components of the earth model that are relevant to the identified inconsistency and then ranking the identified components according to the degree of uncertainty. The component selected is preferably updated using the minimum changes needed to eliminate an identified inconsistency.
Preferably, the received data includes the effective density of the drilling fluid used in the construction of the borehole and one at least other parameter, and the selected component is updated by using the effective density and at least one other parameter.
The invention is also embodied in a method for drilling a borehole using an earth model that is updated according to the invention. The invention is also embodied in a computer readable medium that is capable of causing a computer to update the earth model according to the invention.